Laser display apparatus

ABSTRACT

Provided is a laser display apparatus in which speckle can be suppressed. The laser display apparatus includes: a laser comprising a first mirror and a second mirror that are separated by a cavity distance and form a cavity; and a spatial modulator modulating light transmitted through the first mirror of the laser to display an image on a screen, wherein, when a coherence length l c  and an effective reflectivity R are defined by the following equation:  
                     l   c     =     2   ⁢   d   ⁢           ⁢       π   ⁢           ⁢     R     1   /   2           1   -   R                     R   =         R   1     ×     R   2                       [   Equation   ]             
 
where d is the cavity distance of the laser, R 1 , is the reflectivity of the first mirror, and R 2  is the reflectivity of the second mirror, the coherence length l c  satisfies the following inequality:
 
 0 &lt;l c   ≦0.85   [cm].

CROSS-REFERENCE TO RELATED PATENT APPLICATION

Priority is claimed to Korean Patent Application No. 10-2005-0123160,filed on Dec. 14, 2005 in the Korean Intellectual Property Office, thedisclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The present disclosure relates to a laser display apparatus having alaser light source, and more particularly, to a laser display apparatusin which a speckle can be suppressed.

2. Description of the Related Art

Generally, laser display apparatuses can be easily applied to projectiondisplays due to the collimation of a laser beam. Moreover, the intensityof a laser beam is greater than the intensity of light generated fromother light sources, and thus a laser can be used to realize a clear,large screen image.

A laser display apparatus uses a laser with high coherency, causingspeckle. Speckle refers to coherent noise randomly occurring due to highcoherency of a laser light source.

For example, when an image illustrated in FIG. 1A is to be realized on ascreen by a laser display apparatus, the image is realized asillustrated in FIG. 1B due to the above described speckle.

U.S. Pat. No. 5,274,494 discloses a “Speckle Suppression Illuminator”which purports to suppress the above described speckle.

The “Speckle Suppression Illuminator” includes a Raman cell, which is achamber storing a gas for dispersing incident light, in front of lightsources radiating coherent light with a first wavelength. Accordingly,the illuminator provides a spatially coherent and temporally incoherentlaser beam to reduce the intensity and occurrence area of the speckle.

However, since a Raman cell is in the path of the light radiated fromthe laser light source, the optical structure of the laser displayapparatus is complicated and the number of manufacturing processes andcosts are high.

SUMMARY OF THE DISCLOSURE

The present disclosure provides a laser display apparatus in whichspeckle is suppressed without the inclusion of any additional elementsin the optical path.

According to an aspect of the present disclosure, there is provided alaser display apparatus comprising: a laser comprising a first mirrorand a second mirror that are separated by a cavity distance and form acavity; and a spatial modulator modulating light transmitted through thefirst mirror of the laser to display an image on a screen, wherein, whena coherence length l_(c) and an effective reflectivity R of the laserare defined by the following equation: $\begin{matrix}\begin{matrix}{l_{c} = {2d\quad\frac{\pi\quad R^{1/2}}{1 - R}}} \\{R = \sqrt{R_{1} \times R_{2}}}\end{matrix} & \lbrack{Equation}\rbrack\end{matrix}$where d is the cavity distance of the laser, R₁ is the reflectivity ofthe first mirror, and R₂ is the reflectivity of the second mirror, thecoherence length l_(c) satisfies the following inequality:0<l_(c)≦0.85[cm]  [Inequality]

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1A is an original image;

FIG. 1B is the image of FIG. 1A with speckle;

FIG. 2 is a schematic view of a laser display apparatus according to anembodiment of the present disclosure;

FIG. 3 is a graph illustrating coherence length with respect toeffective reflectivity of a laser;

FIG. 4 illustrates how speckle contrast (Cs) is measured in a laser;

FIGS. 5A through 5D are photographs of images at effectivereflectivities (R) of 90.5%, 66.6%, 18.7%, and 5.4%, respectively;

FIG. 6 is a graph of speckle contrast (Cs) with respect to effectivereflectivity; and

FIG. 7 is a graph of speckle contrast (Cs) with respect to coherencelength.

DETAILED DESCRIPTION OF THE DISCLOSURE

The present invention will now be described more fully with reference tothe accompanying drawings, in which exemplary embodiments of theinvention are shown.

FIG. 2 illustrates the optical arrangement of a laser display apparatusaccording to an embodiment of the present disclosure. Referring to FIG.2, the laser display apparatus scans laser light onto a screen 30, andincludes a laser 10, and a spatial modulator 20 modulating incidentlight to display an image on the screen 30.

The laser 10 includes first and second clad layers 11 and 15, an activelayer 13 interposed between the first and second clad layers 11 and 15,and first and second mirrors 12 and 14 forming a cavity on a lateralside of the active layer 13 and the first and second clad layers 11 and15 in this exemplary embodiment. The laser 10 radiates visible lightwith a wavelength ranging from about 400 nm to about 700 nm. The firstmirror 12 and the second mirror 14 are separated from each other by acavity distance d.

When power is supplied to the laser 10, a beam generated by the activelayer 13 is resonated between the first mirror 12 and the second mirror14 and only light with a predetermined wavelength is amplified andtransmitted through the first mirror 12 and the second mirror 14,

The spatial modulator 20 modulates the light transmitted through thefirst mirror 12 to display the image on the screen 30,

In the present embodiment, the speckle contrast on the screen 30 isreduced by altering a coherence length l_(c) by controlling the cavitydistance d and effective reflectivity R of the laser 10, Therelationship between the laser 10 and the coherence length l_(c), whichdetermines the speckle intensity on the screen 30, will now bedescribed, and then the effective reflectivity R of the laser 10 and theoptimisation of the cavity distance d will be described.

The coherence length l_(c) with a narrow width equal to the width of thelaser 10 can be expressed by Equation 1. $\begin{matrix}\begin{matrix}{l_{C} = \frac{cF}{v_{F}}} \\{= {2{dF}}} \\{= {2d\quad\frac{\pi\quad R^{1/2}}{1 - R}}}\end{matrix} & \left\lbrack {{Equation}\quad 1} \right\rbrack\end{matrix}$

where c is the speed of light, F is the finesse in a Fabry-Perot cavity,V_(F) is the number of modes for a given frequency, d is the cavitydistance, and R is the effective reflectivity of the laser 10 defined bythe following Equation 2.R=√{square root over (R₁×R₂,)}  [Equation 2]

where R₁ is the reflectivity of the first mirror 12 and R₂ is thereflectivity of the second mirror 14,

Referring to Equation 1, the coherence length l_(c), which denotes thedistance over which coherence is maintained, is dependent on the cavitydistance d and the effective reflectivity R of the laser 10,Accordingly, as the effective reflectivity R and the cavity distance dare reduced, the coherency of the radiation beam is reduced and thus thespeckle intensity on the screen 30 is reduced.

FIG. 3 is a graph illustrating the coherence length with respect toeffective reflectivity of a laser. In detail, FIG. 3 illustrates thecoherence length l_(c) and the coherence factor F_(c) of GaN type laserswith effective reflectivities of 90.5%, 66.6%, 18.7%, and 5.4%,respectively.

Referring to FIG. 3, as the effective reflectivity of the laserdecreases, l_(c) and F_(c) decrease.

FIG. 4 is a schematic view of a device measuring the speckle contrast ofa laser. The speckle contrast measuring device includes a GaN type laser41 with effective reflectivities of 90.5%, 66.6%, 18.7%, and 5.4%,respectively, and a cavity distance of 650 μm mounted on a coolingdevice 43, The laser 41 is driven by a laser driving source 45 and afar-field of the laser 41 is projected on a screen 47 without a lenssystem. The image on the screen 47 is photographed with a camera 49,e.g. a digital camera with a manual function, with an F10 aperture and−2.0 EV exposure, and the speckle contrast Cs is calculated usingEquations 3 through 5. $\begin{matrix}{{C_{S} = \frac{\sigma}{avg}}{where}} & \left\lbrack {{Equation}\quad 3} \right\rbrack \\{{\sigma = \sqrt{\frac{\sum\limits_{n = 1}^{N}{{{avg} - {I(n)}}}^{2}}{N}}}{and}} & \left\lbrack {{Equation}\quad 4} \right\rbrack \\{{avg} = \frac{\sum\limits_{n = 1}^{N}{I(n)}}{N}} & \left\lbrack {{Equation}\quad 5} \right\rbrack\end{matrix}$

where l(n) is light intensity, N is the number of data, avg is theaverage light intensity, and σ is the standard deviation.

FIGS. 5A through 5D are photographs of images taken at effectivereflectivities of 90.5%, 66.6%, 18.7%, and 5.4%, respectively. FIG. 6 isa graph of speckle contrast Cs with respect to effective reflectivity.

FIG. 5A is a photograph of an image projected onto a screen by a laserin which the reflectivity R₁ of the first mirror 12 and the reflectivityR₂ of the second mirror 14 are both 90.5%. The speckle contrast Cs is0.422 and, as illustrated, the image has a rough speckle.

FIG. 5B is a photograph of an image projected onto the screen by a laserin which the reflectivity R₁ of the first mirror 12 is 66.6% and thereflectivity R₂ of the second mirror 14 is 90.5%. The speckle contrastCs is 0.312 and the speckle is relatively smaller than in FIG. 5A.

FIG. 5C is a photograph of an image projected onto the screen by a laserin which the reflectivity R₁ of the first mirror 12 is 18.7% and thereflectivity R₂ of the second mirror 14 is 90.5%. The speckle contrastCs is 0.171 and the speckle is much smaller than in FIGS. 5A and 5B.

FIG. 5D is a photograph of an image projected onto the screen by a laserin which the reflectivity R₁ of the first mirror 12 is 5.4% and thereflectivity R₂ of the second mirror 14 is 90.5%. The speckle contrastCs is 0.133 and the image has barely any speckle.

As described above, as the reflectivity R of the laser is reduced from90.5% to 5.4%, the coherency is decreased and, as illustrated in FIG. 6,the speckle contrast Cs can be reduced by as much as one third.

Meanwhile, the coherence length l_(c) does not only depend on theeffective reflectivity R but also on the cavity distance d as shown inEquation 1. Also, the coherence length l_(c) which is defined by thecavity distance d and the effective reflectivity R is related to thevariation of the speckle contrast Cs.

FIG. 7 is a graph of speckle contrast with respect to coherence lengthl_(c). Referring to FIG. 7, as the coherence length l_(c) increases, thespeckle contrast Cs increases, leveling out at high coherence lengths.For example, when l_(c) is 0.85 cm, Cs is 0.25 or less, and when l_(c)is greater than 0.85 cm, Cs is greater than 0.25.

Accordingly, the laser can be improved without additional opticalelement or a radio frequency (RF) driving device.

That is, referring to FIG. 2, the laser 10 has a coherence length l_(c)which satisfies the following inequality.0<l_(c)≦0.85[cm]  [Equation 6]

The above inequality is determined according to the relationship betweenthe coherence length l_(c) and the speckle contrast Cs, and when thecoherence length l_(c) exceeds 0.85 cm, the speckle contrast Cs exceeds0.25, and thus the speckle on the screen becomes large.

The laser 10 comprises a semiconductor laser or a diode pumped solidstate (DPSS) laser. If the laser 10 comprises a semiconductor laser, thecavity distance d may satisfy the following inequality whilesimultaneously satisfying Equation 6.100≦d≦5000 [μm ]  [Equation 7]

If the laser 10 comprises a diode pumped solid state (DPSS) laser, thecavity distance d may satisfy the following inequality whilesimultaneously satisfying Equation 6.1≦d≦50 [mm]  [Equation 8]

As described above, the laser display apparatus can suppress speckle ona screen without the inclusion of any additional components bycontrolling the effective reflectivity and/or cavity distance in thelaser and thus optimising the coherence length l_(c).

While the present disclosure has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present disclosure as defined by the following claims.

1. A laser display apparatus comprising: a laser comprising a firstmirror and a second mirror that are separated by a cavity distance andform a cavity; and a spatial modulator modulating light transmittedthrough the first mirror of the laser to display an image on a screen,wherein, when a coherence length l_(c) and an effective reflectivity Rof the laser are defined by the following equation: $\begin{matrix}\begin{matrix}{l_{c} = {2d\quad\frac{\pi\quad R^{1/2}}{1 - R}}} \\{R = \sqrt{R_{1} \times R_{2}}}\end{matrix} & \lbrack{Equation}\rbrack\end{matrix}$ where d is the cavity distance of the laser, R₁ is thereflectivity of the first mirror, and R₂ is the reflectivity of thesecond mirror, the coherence length l_(c) satisfies the followinginequality:0<l_(c)≦0.85 [cm]
 2. The laser display apparatus of claim 1, wherein thelaser comprises a semiconductor laser.
 3. The laser display apparatus ofclaim 2, wherein the cavity distance d satisfies 100 ≦d ≦5000 [μm ]. 4.The laser display apparatus of claim 1, wherein the laser comprises adiode pumped solid state laser.
 5. The laser display apparatus of claim4, wherein the cavity distance d satisfies 1 ≦d ≦50 [mm].
 6. The laserdisplay apparatus of claim 1, wherein the laser radiates visible lightwith a wavelength ranging from about 400 nm to about 700 nm.